Off topic: Help needed for Vendor specific USB

I need some help from the more advanced programmers to implement vendor specific (0xFF class) USB device code using the libopencm3 library.
I didn't find any examples for it. I found code for DFU, HID, MSC, and they all work, but I simply couldn't write a custom USB device.

The documentation of libopencm3 doesn't worth s..t. I can't imagine how you could use it at all!